Experimental Study of Dewetting Instability in Polymer Multilayer Shear Flow

The interfacial stability of multilayer Couette flow is investigated experimentally. A better understanding of this flow is critical for the processing of multilayer polymeric structures, such as multilayer coextrusion. We first look at the liquid film rupture predictions from numerical simulations of the thin film equation that considers capillary and van der Waals forces. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the regime of low shear rates the rupture is delayed when compared to the no-shear problem, while at higher shear rates it is suppressed. Then, experiments investigating the effect of shear on the dewetting of thin polymer films at different temperatures by using an optical microscope coupled with a shearing hotstage will be presented. The dewetting dynamics, i.e. the growth of dewetting holes is monitored over time at various shear rates. It is observed that their circularity is modified by shear and that for all temperatures and thicknesses studied, the growth speed of the formed holes rapidly increases with increasing shear rate. A model balancing capillary forces and viscous dissipation while taking into account shear-thinning is then proposed and captures the main features of the experimental data, such as the ellipsoid shape of the holes and the faster dynamics in the direction parallel to the shear.